The prime mover or engine of a heavy piece of construction equipment or the like normally has an output that rotates at a constant speed or whose speed at best can be varied within a predetermined relatively small range. Thus it is standard practice to provide a variable-speed drive between the output of such a prime mover and the load to be operated, which can be the wheels of the vehicle, a winch, or another peripheral apparatus.
A hydromechanical transmission is described in S.A.E. Publication 72 07 24 of September 1972. It has two forward speeds and a reverse speed, and comprises a hydraulic transmission and mechanical transmission. The hydraulic transmission comprises a pair of hydraulic machines, a pump and a motor, one of which is of variable volume. The mechanical transmission comprises a four-shaft planetary-gear transmission. This arrangement is set up for load splitting so that it is possible with a limitedly variable or even fixed speed of the input shaft of the drive to obtain a steplessly variable speed at the output shaft.
In this arrangement in the low forward speed and in reverse the drive is purely hydrostatic, that is there is actually no load splitting and all of the force transmitted from the input shaft to the output shaft of the drive is transmitted through the hydrostatic pump-and-motor assembly. Obviously the force-transmitting capacity of such a drive is limited at least in these two low-speed ranges by the capacity of the hydrostatic assembly.
Thus it is necessary in such a system either to provide it with an extremely heavy-duty pump-and-motor assembly, or to reduce the maximum speed both in forward low gear and reverse gear so that the product of speed times torque, which determines the drive capacity, remains low and within the range of the hydrostatic subassembly. For this reason the drive will either be quite expensive as a result of having a very large hydrostatic pump-and-motor assembly, or will only provide a very slow low gear and reverse.
Another disadvantage of this system is that the vehicle jumps when started in forward and reverse. This action is the result of the inertia of the pump-and-motor assembly. An axial-piston pump is normally employed and is normally constantly driven by the input shaft of the transmission. Thus when the output shaft of the drive should stand still the swash plate of the pump is aligned perpendicular to the axis of the pump so that even though the pump is driven by the input shaft, it will not act as a pump and the motor of the hydrostatic subassembly will not operate. Only when the swash plate has been tipped through a relatively great angle will the inertia of the motor be overcome so that it will start rotating and, hence, will drive the output shaft. As a result jumpy starting and stopping is taken for granted with such a drive.